Prince presets an award on stage at the 57th Annual Grammy Awards in Los Angeles February 8, 2015. (ROBYN BECK/AFP/Getty Images)

Prince presets an award on stage at the 57th Annual Grammy Awards in Los Angeles February 8, 2015.

*Toxicology tests reveal that Prince died of an opioid overdose, the Associated Press reports, citing a law enforcement official.

The official, who is close to the investigation, spoke on condition of anonymity because he was not authorized to speak to the media.

Prince had been treated for a drug overdose six days before his death on April 21, according to TMZ. On April 15, his private jet had made an emergency landing in Moline, Illinois, hours after he performed in Atlanta. The singer was reportedly given a “save shot” at the time to counteract the effects of an opiate.

The Daily Mail spoke to a man, “Doctor D,” who claimed to be Prince’s supplier of opioids. He said the artist “would spend up to $40,000 a time on six-month supplies of Dilaudid pills and Fentanyl patches – both highly addictive opioid pain killers.”

The day before Prince died, his team called prominent opioid addiction specialist, Dr. Howard Kornfeld, seeking urgent assistance. Unable to get there immediately from his California office, he sent his son, Andrew Kornfeld, on a redeye flight to Minnesota with the task of assessing Prince’s health and encouraging him to enter treatment for pain management and potential addiction issues, according to reports.

But by the time Andrew Kornfeld arrived at the singer’s Paisley Park complex on the morning of April 21, it was too late. He and two Prince representatives found the 57-year-old entertainer unresponsive in an elevator.

Investigators have been looking into whether Prince died of an overdose and whether a doctor was prescribing him drugs in the weeks before his death.